Frequency modulator



March 28, 1944. M. e. CROSBY 2,345,101

FREQUENCY MODULATOR Filed March 27. 1942 MODULA TION INPUT STABLE OSCILLATOR F 1 f Fs Mao Y R Y Z m ay/7 v n M Ma? Y BY ){4 A STAALE OSCILLATOR Flf Fs 7 ULA rlolv I/v/wr Pa'tented- Mar. 28, 1944 2.34am summer momma-ma Murray G, Crosby, Riverlread, N.

Radio Corporation of America,

Delaware Y., usiznor to a corporation of Application March 21, masuai No. 436,437 9 Claims. (01. ire-111.5)

This application concerns a new and improved frequency modulator in which a pair of. heterodyne converters and a stable oscillator are caused to oscillate and produce wave energy of a low frequency and wave energy of a high frequency. The low frequency is the difference between the stable oscillator frequency and the frequency of the high frequency wave energy. The high frequency is the sum or difierence of the low frequency and the stable oscillator frequency. The stability of this oscillating system lies in the low frequency circuit since it has the most rapid variation of phase with frequency. That is to say, if we assumed the low frequency is caused bysome electrical force to drift or change, the extent of drift or change in phase is a greater fraction of the low frequency than would be the case if the same force caused a high frequency to drift or change to the same extent. Moreover, it is easier to obtain high selectivity and as a consequence, greater stability in a circuit operating at low frequency than in a circuit operating at higher frequency.

For frequency modulation, the tuning of this low frequency circuit is modulated by means of reactance tubes or by means of a balanced oiftuned system arranged for differential modulation.

In the prior description and in the description which follows I describe my system in terms of frequency modulation. It will be understood, however, that the system may be operated as a phase modulator by modifying the modulation or control potentials to make the amplitude of the said modulation or potentials substantially proportional to their frequency so that the frequency of the oscillations is not proportional to the modulation or control potential amplitude whereas the phase deviations are proportional to the modulation or control potential amplitude. The term frequency modulation as used herein is to mean frequency modulation or phase modulation or modulation having the characteristics of frequency and phase modulation.

There is a type of prior art frequency modulator which bears some resemblance to the one described herein. That type is the system invented by C. W. Hansell in which a low frequency oscillator wherein good stability can be obtained is frequency modulated and heterodyned to the radiation frequency as disclosed in Hansell U. S. Patent No. 1,819,508. The process of heterodyning is used so that stability is maintained since by heterodyning, the frequency variations or drifts of the low frequency oscillator are not multiplied. In the system of this application, the low frequency and high frequency circuits are arranged in a self-oscillating pair of converters so that generation of the high frequency is dependent upon re-entrance through the low frequency circuits. This gives the high frequency circuit a stability in cycles which is the same as the low frequency circuit.

I describing my invention, reference will be made to the attached drawing wherein Figs. 1 and 2 show two forms of my new and improved frequency modulation system.

Fig.1 shows a specific embodiment of the system. Tubes l8 and iii are heterodyne converters which both have one of their grids fed by stable oscillator 50. The oscillator 50 feeds oscillations to the grid 9 of tube l0 and the grid 29 of tube to. The anode H of tube ill is coupled to the tuned circuit 28 and by coupling condenser 3! to the grid 35 of tube 40. The anode 43 of tube is coupled to tuned circuit 41 and by coupling condenser 49 and lead I3 to thegrid l5 of tube to. The tubes ill and 40 act as detectors or mixers and also amplify the voltages translated therein. Tuned circuit 26, which couples the plate circuit of converter III to the grid circuit of converter 40, is tuned to the low frequency, f. Tuned circuit 41, which couples the plate circuit of converter 40 to the grid circuit of converter i0 is tuned to the high frequency F. The stable oscillator has the frequency Fs=F:f.

The start of oscillations in this circuit is caused by the same sort of impulse that usually starts the ordinary type of oscillator. A surge of current or a noise voltage causes an alternating curent voltage to appear somewhere in the system, say, across tuned circuit 26. This voltage is fed to the grid 35 of converter tube 40 where it heterodynes with alternating current from the stable oscillator 50 to produce an output frequency which is the diflerence between ,1, the frequency to which 26 is'tuned, and the stable oscillator frequency Fs (where Fs equals F+j), or F. (Circuit 41 "is tuned to the sum frequency when Fs=F-l.) The frequency F is fed to the grid of converter tube Ill where it heterodynes with alternating current from the stable oscillator 50 to produce the frequency f in circuit 26 again. Amplification in the converters Ill and 4|! builds up the voltage so that each time it traverses the circuit it is-larger until the tubes begin to limit the amplitude.

The selectivity and Q of the low frequency circuit 26 determine the stability of the system to a major extent since this circuit has the greatest selectivity. Consequently, the output frequency F has a stability which is dependent upon the stability of the stable oscillator and upon the sharpness of tuned circuit 26.

In order to frequency modulate the oscillating converters, push-pull reactance tubes 60 and 80 are employed. Single reactance tubes may also be used, but the push-pull type has improved stability and other advantages described in my U. S. Patent No. 2,250,095, dated July 22, 1941, U. S. application, Ser. No. 312,446, filed January 5, 1940, now U. 8. Patent No. 2,279,660, dated Apr. 14, 1942, and others. Tube 66 is a capacitive reactance tube having feedback phase shifter C, R. The reactance of C is normally made large compared to the resistance of R. The alternating current voltage in circuit 26 is across the output impedances of tubes 60 and 80 and the reactive effect provided by said tubes is in parallel with circuit 26. The anodes 6| and 8| are tied together and the alternating currents thereon are of like phase. The network C and R relatively advances the phase on the grid 63 about 90 so that the grid voltage, and in turn the plate current, leads the anode voltage to get the capacitive reactance effect. Tube 80 is an inductive reactance tube having phase shifter RI, CI. The resistance of RI is normally made large compared to the reactance of CI. The network CI, RI, excited through coupling condenser 85, applies alternating current to grid 83 which is retarded or delayed about 90 so that the plate current in tube 80 lags the plate voltage to provide the inductive reactance effect. Transformer 66 differentially modulates the two tubes so that their reactive effects are accumulative.

The application of frequency modulation to the tuning of tuned circuit 26 has an effect which is the same as that which is produced when an ordinary oscillator is modulated by a reactance tube. That is, the variation applied by the reactance tube may be looked upon as the same as the variation in tuning of the ordinary oscillator. The selectivity of tuned circuit 41 is broad compared to the degree of frequency variation it passes so that for all practical purposes, the oscillation characteristics are determined by the low-frequency tuned circuit 26. The phase is not changed when 26 is detuned by the reactance tube sincethe oscillations immediately assume the new frequency determined by the amount of detuning.

Fig. 2 shows an embodiment in which two sets of converter tubes MI and I02 are arranged with their low frequency circuits I05 and I06 oiftuned to opposite sides of the unmodulated or carrier frequency (intermediate frequency 1) so that differential modulation of the two sets will cause the frequency to be modulated between the limits dependent upon the amount of ofituning of the circuits. Converter tubes I03 and I04 have a common output circuit I01 which is tuned to the highfrequency F. Converter tubes IN and I02 have tuned circuits I05 and I06 coupled to the anodes I03 and respectively. These circuits are off-tuned to opposite sides of the unmodulated carrier frequency. The stable oscillator I08 is coupled by lead H to .the grids II! and II! of tubes "I and I02 and by leads I23 to grids I25 and I21 of tubes I03 and Ill. The tuned circuit III is coupled by condensers H2 and H3 to the grids ISI a d "3 of tubes MI and I02. As a consequence, the two converters IM and I02 are fed by the high frequency output from I01 and by the stable oscillator I08 while converters I03 and I 04 are fed by the outputs of circuits I05 and I06 and by .oscillations from I08.

If tube I02 were switched off, it is apparent that circuits I05 and I0! would determine the oscillation frequency. Likewise, if tube IOI is switched oil, circuits I06 and N1 determine the frequency. Thus, if converters IM and I02 are differentially modulated as shown by transformer I09, coupled by resistance I35 and I3! to grids I3I and I33, the frequency fed to converters I03 and I00 is modulated between the two frequencies to which I05 and I06 are tuned. Due to the heterodyne arrangement, the output frequency in I01 is modulated the same amount.

What is claimed is:

1. In a wave length modulation system, two converter tubes each having input and output electrodes, connections for impressing oscillations of a first frequency on the input electrodes of each tube, a circuit coupling the output electrodes of one tube to the input electrodes of the other tube, a stable circuit coupling the output electrodes of said other tube to the input electrodes of said one tube, said couplings and tubes and circuits forming a self-oscillating system, means for tuning said stable circuit to a second frequency, means for tuning said first circuit to a sideband frequency resulting from the mixing in said one tube of oscillations from said source and voltage fed by said stable circuit to said one tube, and means for varying the tuning of one of said circuits in accordance with signals.

2. In a wave generator system, a source of oscillations of constant frequency, a first means for heterodyning and converting currents, a second means for heterodyning and converting currents, coupled to the output of said first means and to said source of oscillations, a coupling between the output of said second means and the input of said first means and a coupling between said source and the input of said first 'means so that self-sustained oscillations are produced.

3. In a wave length modulation system, a first circuit tuned to a first frequency, a source of oscillations of a second frequency, an electron discharge wave mixing and amplifying device having input electrodes coupled to said source and having output electrodes coupled to said first circuit, a second wave mixing and amplifying discharge device having input electrodes coupled to said source of oscillations and to said first circuit and having output electrodes, a second circuit tuned to the frequency of cne of the sidebands produced by combining oscillations from said source with oscillations of the frequency to which said first circuit is tuned, connections between said second circuit and said output electrodes of said second device, a coupling between said second circuit and the input electrodes of said first device, and means for varying the reactance of said first circuit in accordance with signals.

4. In a wave generator and wave length modulation system, a source of oscillations of nearly constant frequency, a first means for heterodyning and converting currents, a second means for heterodyning and converting currents, coupled to the output of said first means and to said source of oscillations, a coupling between the output of said second means and the input of said first means, a coupling between said source and the input of said first means so that said selfsustained oscillations are produced and means for modulating the wave length of the produced oscillations.

5. In a wave length control system, two converter tubes each having input and output electrodes, a source of oscillations of a first and substantially constant frequency, connections between said source and the input electrodes of each of said tubes, a circuit tuned toa second frequency coupled to the output electrodes of one of said tubes, a coupling between said last named circuit and the input electrodes "of the other of' said tubes, a circuit tuned to a third frequency coupled to the output electrodes of said other of said tubes, a coupling between said last named circuit and the input electrodes of said one of said tubes, and means for varying the tuning of one of said tuned circuits in accordance with control potentials.

6. In a wave length modulation system, two converter tubes each having an anode, a cathode and a plurality of control electrodes, a source of oscillations of a first and substantially constant frequency, connections between said source and a control electrode and the cathode of each of said tubes, a circuit tuned to a second frequency coupled to the anode and cathode of one of said tubes, a coupling between said tuned circuit and another control electrode of the other of said tubes, a circuit tuned to a third frequency coupled to the anode and cathode of said other of said tubes, a coupling between said last named circuit and a control electrode of the said one of said tubes, and means for varying the tuningof one of said tuned circuits in accordance with control potentials.

'7. In a wave length modulation system, two

converter tubes each having an anode, a cathode and a plurality of control grids, a circuit tuned to a first frequency, a circuit tuned to a second frequency, a source of oscillations of a substantially constant frequency which is equal to the sum or difference of the said first and second frequencies, connections between said source of oscillations and a control grid and the cathode of each of said tubes, connections including one of said tuned circuits coupling the anode and cathode of one 01' said tubes to a control grid and cathode of the other of said tubes, connections including the other of said tuned circuits coupling the anode and cathode of said other of said tubes to a control grid of said one of said tubes, and means for varying the tuning of said tuned circuit included in said first mentioned connections in accordance with signals.

8. In a wave length modulation system, a first converter tube having an anode, a cathode and a plurality of controlv electrodes, a second converter tube having an anode, a cathode and a plurality of control electrodes, a source of oscillations of a first and substantially constant frequency, a circuit for applying oscillations from said source between a control electrode and the cathode of each of the tubes, a circuit tuned to a second frequency coupled to the anode and cathode of one of said tubes,- a coupling between said tuned circuit and a control grid and cathode of the other of said tubes, a circuit tuned to a third frequency coupled to the anode and cathode of said other of said tubes, a coupling between said last named tuned circuit and a control electrode and cathode of said one of said tubes, and connections to said circuit tuned to said second frequency for controlling the tuning thereof in accordance with signals.

9. In a wave length modulation system, a first pair of converter tubes each having an anode, a cathode and a plurality of control electrodes. a second pair of converter tubes each having an anode, a cathode and a plurality of control electrodes, a source of oscillations of a first and substantially constant frequency, a circuit for applying oscillations from said source between corresponding control electrodes and the cathodes in each of the tubes of said pairs of tubes, the oscillations applied to the said corresponding control electrodes of the tubes in the pairs of tubes being of like phase, two circuits, tuned one above and the other below a second frequency, coupledeach with the anode and cathode of the respective tubes of one of said pairs of tubes, separate couplings between said two circuits and a control electrode and the cathode of the respective tubes of the other pair of tubes, a circuit tuned to a third frequency coupled between the anodes and cathodes of said last named other pair of tubes, a coupling between said last named circuit and a control electrode of each of the tubes of said first named pair of tubes, and connections for difierentially modulating corresponding control electrodes of the tubes of said first named pair of tubes in accordance with signals.

MURRAY G. CROSBY. 

